In thermophotovoltaic (TPV) energy conversion, infrared light radiated by a heat source is converted to electricity using a semiconducting junction. To convert infrared light efficiently, semiconductors with low bandgaps are required. Bandgaps in the range of 0.55 eV to 0.74 eV respond to parts of the infrared light spectrum that have practical commercial application. The bandgap of 0.55 eV is targeted for most low-temperature TPV systems.
Large area single crystals used in semiconductors cannot be grown in bulk form at these bandgaps. Ternary compounds, such as InGaAs, can be grown by gas-phase processes to produce layers within the desired bandgap range, i.e., between 0.55 eV to 0.74 eV. To date, there are no known substrates with the same lattice constants as the desired InGaAs composition. Hence, lattice mismatched compositions are grown. The resulting strain due to lattice-mismatched layers typically relaxes by generating a high defect concentration that causes less than optimal device performance.
In the past, the use of quaternary growth methods, such as in the case of GaInAsSb deposited on GaSb substrates, have made it possible to manipulate the bandgap of the epitaxial layer independently from the lattice constant. As a result, lattice-matched, relatively dislocation-free layers have been grown at the desired bandgap. However, quaternary growth systems are very complex and difficult to implement on a commercial scale.
A critical composition buffering technique as first described in V. Krishnamoorthy, P. Ribas, and R. M. Park, Appl. Phys. Lett. 58 (1991) pg. 2000, was later applied to the growth of InGaAs on GaAs substrates using molecular beam epitaxy (MBE) as the growth technique. V. Krishnamoorthy, Y. W. Lin, and R. M. Park, xe2x80x9cApplication of xe2x80x98critical composition differencexe2x80x99 concept to the growth of low dislocation density InGaAs on GaAsxe2x80x9d, J. Appl. Phys., 72 (1992) pg. 1752.
In critical composition buffering, the dislocation density in the epitaxial layer is dependent on the compositional difference between the epitaxial layer and the substrate. In the case of a single layer comprising InxGa1xe2x88x92xAs grown on GaAs, when x is kept below x=0.18, the epitaxial layer is dislocation free. In other words, when the relative concentration of In between the epitaxial layer and the substrate is kept below 18%, the dislocation density in the epitaxial layer remains below 10 cmxe2x88x922.
Dislocations that are observed in the GaAs substrate do not propagate into the epitaxial layer. However, when the critical composition is exceeded (x greater than 0.18), dislocations are observed in the epitaxial layer. It is thought that by controlling the relative composition between layers, the underlying layer, (i.e., the substrate, for example) has a lower yield strength than the layer being deposited. Therefore, dislocations created due to lattice-mismatch predominately propagate into the softer material. The strain caused by lattice mismatch is relieved and the top layer is relatively dislocation free.
If the composition ultimately desired has a higher In concentration than the critical composition difference, additional buffering layers can be used. Each layer in the buffer structure will have an In content less than the critical composition difference with respect to the previous layer.
Determining the composition of each layer is not straight forward. The important composition is not the actual alloy composition, but the xe2x80x9ceffectivexe2x80x9d composition. That is, due to incomplete lattice relaxation, the epitaxial layer will appear to have a composition different from the actual composition when the composition is inferred from x-ray diffraction measurement of lattice parameter. Therefore, to successfully employ the critical composition buffering technique, the effective composition of each layer is measured. The composition of the subsequent layer can then be determined. Effective composition can be measured by interrupting growth after each layer and measuring its lattice parameter (effective composition) and actual composition with x-ray diffraction and other analytical techniques.
There is a limit to the ultimate composition that can be grown dislocation free. This limit is determined by how the yield strength varies with composition. In the case of InxGa1xe2x88x92xAs, yield strength increases with In concentration up to a maximum of x=0.5. As the In is increased beyond this level, successive layers are weaker and critical composition buffering does not work.
In order to achieve a desired bandgap for the InGaAs epitaxial layer, it is important to develop an appropriate buffer structure. A proper buffer structure is necessary to accommodate the lattice strain produced when growing InGaAs compositions with different lattice constants than the substrate. These alloys are said to be xe2x80x9clattice mismatchedxe2x80x9d to the substrate. Lattice strain causes defects in the semiconductor. The defects are electrically active and degrade electrical performance. In photovoltaic devices, this degradation is observed as an increased reverse saturation current or dark current and reduced minority carrier diffusion lengths. A buffer that could accommodate the strain thereby preventing defects in the overlying active layers is highly desirable.
U.S. Pat. No. 5,387,796 to Joshi et al. suggests the use of buffer layering in connection with growing InGaAs compositions over the InP substrate. The buffer disclosed in Joshi et al. is comprised of InAsP, deposited in discreet layers. Each layer has a distinct composition which varies only slightly from that of the previous layer. The layers are deposited by hydride vapor phase epitaxy. The InAsP composition of each subsequent layer in the buffer must be such as to produce a lattice mismatch of 0.13% or less relative to the preceding layer. After the growth of the InAsP buffer, the InGaAs active layer is grown and doped with sulfur to a specified concentration. After growth of the entire structure, the wafer is thermally cycled to further improve the quality of the InGaAs layer.
The need in Joshi et al. for a mismatch of 0.13% or less requires a large number of discreet buffer layer steps. For example, in a situation where a 0.6 eV InGaAs alloy is desired, the method of Joshi et al. requires eight intervening InAsP buffer layers between the InP substrate and the 0.6 eV InGaAs alloy. The manufacturing process is relatively cumbersome and lengthy with the requirement of small mismatch values and of so great a number of buffer layer steps. This manufacturing process further calls for additional steps such as doping, thermal annealing or thermal cycling to reduce leakage current (i.e., dislocations) in the resulting photodectector.
In order to simplify the manufacturing process for producing InGaAs layers on InP substrates, there is a need to provide a buffering scheme that both eases the manufacturing process and reduces or eliminates the need for doping or heat treatment.
The present invention is directed to the use of a novel buffering scheme for growing InGaAs on an InP substrate. The method is based on the critical composition difference technique and the yield strength characteristics of the InAsP alloy system.
A method for growing InGaAs epitaxial layer on a lattice mismatched InP substrate calls for depositing, by an epitaxial growth process at least one layer of InAsyP1xe2x88x92y over an InP substrate to provide a buffer. Each succeeding InAsyP1xe2x88x92y buffer layer has a distinct composition which produces less than a critical amount of lattice mismatch relative to the preceding layer. Each InAsyP1xe2x88x92y layer has a lattice-mismatch between 0.26% and 1.3% relative to the previous layer. An InxGa1xe2x88x92xAs epitaxial layer is grown over the buffer wherein 0.53xe2x89xa6xxe2x89xa60.76. The desired InGaAs alloy is grown lattice-matched to the uppermost InAsP buffer layer. The resulting InGaAs epitaxial layer has a bandgap in the range of about 0.55 eV to about 0.74 eV.
A resulting photovoltaic InGaAs structure comprises an InP substrate with at least one InAsP buffer layer sandwiched between the substrate and the InGaAs epitaxial layer. The buffer layer has a critical lattice mismatch between 0.26% and 1.3% relative to the InP substrate. Additional buffer layers will likewise have a lattice mismatch between 0.26% and 1.3% relative to the preceding layer. The number of buffer layers is determined by the resulting bandgap desired in the InGaAs epitaxial layer, which, in turn, determines the composition of the InxGa1xe2x88x92xAs epitaxial layer.
An advantage of the present invention is that only a small number of buffer layers are required prior to the deposition of the InxGa1xe2x88x92xAs. This results from the high level of lattice mismatch. In case of a resulting bandgap of 0.6 eV wherein x=0.68, only two buffer layers are needed. This simplifies the manufacturing process and is, hence, an economical improvement over the prior art. The fewer layers that are required leads to shorter durations in the reactor where the layers are deposited. There are savings in manufacturing time.
Another advantage of the present invention is that the relative thickness of each buffer layer is beyond a critical thickness at which point the strain caused by lattice mismatched is 95-99% relaxed. Hence, there is little or no long term degradation of the resulting photovoltaic cell. The cell offers good performance capabilities.
Superior device performance is realized in the present invention compared to devices fabricated using compositionally stepped, InGaAs buffer layers.
Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.